1. Field of the Invention
This invention relates generally to wear and impact resistant composite bodies such as those used in drilling, cutting or machining hard substances. More specifically, the present invention provides an improved transition zone between a layer of super-hard material and a substrate. The super-hard material in this case is a sintered polycrystalline diamond (PCD) which is fixed to a substrate such as cemented metal carbide composite. The transition zone between the diamond and carbide substrate is an inherently vulnerable area which is often the source of failure of the composite body due to residual stresses created as a result of the manufacturing process. The invention uses the residual stress to benefit the composite body instead of trying to eliminate it.
2. Prior Art
Polycrystalline diamond compacts (PDCs) are diamond layers fixed to substrates. Generally, PDCs provide a hard drilling and cutting surface for use in the mining and machining industries. Specifically, they provide high resistance to wear and abrasion having the strength of diamond and the toughness of a carbide substrate.
Individual layers of the PDC, however, do not share all of the characteristics of the composite body. For example, while polycrystalline diamond is very strong and abrasion resistant, it is not very tough. The quality of toughness is quantified in the measurement of impact resistance. Impact resistance is of vital concern to the oil and natural gas mining industry because of the high impact and high abrasion environments encountered while drilling through various layers of rock.
A harsh working environment is not the only problem encountered by users of PDCs. The conventional process of fixing the polycrystalline diamond to the substrate causes the development of high internal residual stresses between the different layers during high pressure and high temperature formation. These stresses are the result of thermal expansion and modulus differences between the diamond layer and the substrate. Thus, residual stresses can add to the problem of the already low impact resistance of diamond layers.
What is needed is a way to couple the polycrystalline diamond layer to the substrate in such a way as to modify the internal stresses in the transition zone such that they improve the PDC's performance rather than detract from it. Modification of residual stresses in the transition zone can also improve the initial stress state on the cutting edge of the PDC. This provides increased impact resistance, and consequently extends the useful life of the PDC.
The prior art technique for the sintering of diamond and fixing it to a tungsten carbide substrate is demonstrated by U.S. Pat. No. 3,745,623. The transition zone between the PCD and the substrate is abrupt. An abrupt transition zone is inherently weak, especially when the transition layer must withstand stresses up to about 200,000 psi. In general, PDCs have residual interface stresses from formation of about 80,000 to 150,000 psi, making the strength of the interface critical to maintain PDC integrity.
As stated above, modifying residual interface stresses can increase overall PDC strength. One method of increasing the PDC strength is illustrated by U.S. Pat. No. 4,604,106 which teaches, among other things, the use of one or more transition layers composed of mixtures of pre-sintered tungsten carbide and diamond. By varying the percentage of diamond and carbide in the layers, the residual stress is reduced in stages throughout the transition layers. However, one of the drawbacks of this technique is that because the sintering process apparently depends on the migration of liquid cobalt from the carbide substrate into the diamond powder, the transition layers may inhibit this process, resulting in a diamond surface with reduced abrasion resistance.
A different technique for modifying residual stress is disclosed in U.S. Pat. No. 4,784,023 wherein linear grooves in the carbide substrate increase drilling performance. However, the grooving of the substrate was not intended to reduce internal stress in the PDC. The grooves are oriented such that they engage the workpiece face during the drilling operation. This orientation has the effect of making the stress field non-uniform, possibly leading to PDC cracking, especially in a plane parallel to the grooves. In addition, the grooves cause internal stresses of their own due to non-uniform sintering during the high pressure and temperature fixing process. The result is less dense sintered diamond areas. This phenomenon leads to substantial instances of cracking when the cutters are brazed into the bits.
U.S. Pat. No. 4,629,373 appears to get around the problem of stresses at a transition zone between a polycrystalline diamond layer and a substrate by eliminating the substrate. For example, the diamond layer is brazed directly into a tool holder or other support device. However, brazing is a weaker bond than the one created by the high pressure and temperature process used in the present invention to bond the diamond layer to a substrate. Furthermore, without the substrate, the tool cannot be used in high impact or high force situations which a carbide substrate is designed to withstand.
A different approach to the problem is taught in U.S. Pat. No. 5,011,515 where one aspect of the invention is a technique for modifying the topography of the carbide substrate to create a transition zone comprised of carbide and diamond. Specifically, a three dimensional pattern of irregularities on the surface of the substrate taper into the diamond layer are provided in an attempt to spread out the residual internal stresses over a larger surface area to achieve a more impact resistant PDC. However, the irregularities can act as wedges, forcing the diamond and carbide apart.
U.S. Pat. No. 5,351,772, among other things, appears to present a method of modifying the residual stresses through the use of raised carbide lands disposed on the carbide upper surface, over which the diamond is sintered. While the idea of redistributing the stresses through the use if radial lands is beneficial, freedom to optimize stresses is less pronounced than using the projections of the present invention. As will be explained, the ability to vary density, height and location of the projections in the current invention is more pronounced. Furthermore, this prior art appears limited to complete coverage of the lands, whereas the present invention will be shown to allow projections to penetrate the diamond surface, providing highly compressed areas to arrest crack propagation and to allow further load bearing capacity on the top diamond surface.
Finally, U.S. Pat. No. 5,355,969 discloses the use of surface irregularities to reduce residual stress between the polycrystalline diamond layer and the carbide substrate. Specifically, the patent teaches how alternating projections and depressions spaced apart in a radial pattern of concentric circles around the center of the tool can increase the surface area for attachment between the diamond layer and substrate. However, the design is limited to radial patterns, and does not address itself specifically to modifying residual forces in such a way that they increase PDC performance. In addition, the projections are all of equal height, and the depressions of equal depth, doing nothing to manipulate residual stress in a beneficial manner.
In effect, this patent and all those mentioned above focus on spreading out the residual stress over the largest area possible. The major drawback is ignoring the possible benefits that can come from strategically arranging the projections that will result in concentrated residual stress in specific and predetermined areas. Thus, it would be an advantage over the prior art to provide a technique for creating a transition zone between a polycrystalline diamond layer and a carbide substrate that will modify residual stress patterns such that an inherent problem with PDCs can be turned into an advantage. For example, materials under compressive stress can be many times stronger than materials under no stress.
Another problem that has yet to be addressed are the types of stresses on the components of the PDC itself, including, but not limited to the transition zone. Specifically, the carbide layer endures tensile stresses that tend to deform the carbide by pulling the carbide substrate apart, and the diamond layer endures both tensile and compressive stresses which tend to deform the diamond layer by pulling the diamond layer apart in some areas while compressing the diamond layer in other areas. While the compression on the diamond is beneficial, the tensile forces on the diamond and carbide are very detrimental.
It would also be an advantage if the present invention could also strategically modify tensile and compression forces so that the PDC could endure higher loading.